Of all motions experienced on boats, movements about the roll axis are the most troublesome. On very small boats this is experienced immediately when passengers step off the dock onto the boat, as their weight causes a disturbing heel, and then rolling oscillation, of the hull. Even tied to a dock in otherwise calm water, wakes from passing boats can cause unexpected and rapid rolling motions, which cause the boat to slam against the dock, dangerous to boat and passenger alike.
Once the boat is underway, roll presents the most exaggerated and disorienting contrast to the stability of dry land. While pitch (except at very high speed) and heave of the hull generally conform to wave slope and height, roll tends to exhibit a magnification of wave slope. The reason is that the torque generated by the wave forces about the least stable axis of the hull creates an angular momentum which continues the rolling motion after the initial impulse has passed, resulting in heeling angles up to five times greater than wave slope. Moreover, because of the moment generated by the initial roll, the oscillation may continue for some time after the initial impulse has passed. The result is that, of all the motions a boat may exhibit, roll is the least desirable—leaving aside sinking. It is the most uncomfortable and tiring, and one of the greatest causes of motion sickness.
Fortunately, just as rolling motion requires the least energy to initiate, it also takes the least energy to damp, and the most successful boat motion suppression devices have been ones designed to address the roll problem, with most of the effort having been directed toward ships, where the economics justified the effort.
Prior to the early nineteenth century, motive power for boats was primarily sails, which, by their nature, provide a steadying moment—at least, as long as the wind blew. With the advent of steam power and the consequent absence of masts and sails, boat motion control became a more significant concern, and by the late nineteenth century, means were sought to stabilize ships in the roll axis.
The earliest (around 1870) attempts appear to be bilge keels—flat longitudinal plates extending diagonally from the sides of the bottom of the hull. These devices have limited effectiveness unless they are quite large and even then require significant boat speed so that the keels can generate lift by acting as foils.
The first (1880) successful dynamic roll control devices were slosh tanks—an arrangement of water containers inside the hull designed in such a way as to allow a large amount of water (typically 5 to 6% of vessel displacement) to shift from side to side in phase with the roll oscillation so as to damp the rolling impulse. Enhanced versions of this mechanism are used on ships being built at the present time. They are not practical for small boats because of their weight.
Movement of solid weights athwartship were tried briefly at the end of the nineteenth century, but were never considered successful enough to justify further development.
Actively controlled external fins were introduced in about 1925 (in effect, moveable bilge keels) and are the most widely used roll suppression devices on ships today. The fins, usually activated by hydraulic mechanisms, respond to the output of motion sensing devices so as to keep the damping effect of the fin lift in phase with the roll velocity of the vessel. They are generally effective only when the vessel is underway since the passage of water over the fins is necessary in order-for them to generate lift. Active fin systems are capable of stabilizing vessels at rest, but they require very large fins and an even larger energy budget.
Fin stabilizers have found wide application on ships, but not on small boats. One reason why is that ships tend to be underway at cruise speed most of the time when passengers are aboard, as compared to small boats, which are often occupied when at rest or at very low speed. Other reasons for fin stabilizers not being a good roll suppression solution for small boats is that they tend to be expensive, have high appendage drag (at least in planing boats, unless retractable), and are prone to damage from grounding or collision with objects in the water.
Another roll suppression device, used on displacement (but not planing) boats, including commercial fishing craft, is an arrangement of horizontal planing fins, called paravanes, rigged out on cables and booms on either side of the boat, so as to keep a stabilizing force acting on the hull from the lift generated by the planes moving through the water. They tend to be awkward and dangerous, unless used with skill and luck (snagging underwater objects can be nasty), and have found limited use, but at least demonstrate the lengths people will go to prevent boats from rolling. There is a similar system used for stabilizing a boat at rest which employs flat plates (in lieu of the fins) which resist being pulled up through the water column, and thus exert a damping effect in the roll axis. Because of their design, they cannot be used underway.
Gyroscopic roll stabilizers or control moment gyros are another class of devices used for roll suppression. Otto Schlick was the first to develop them, in 1906 (U.S. Pat. No. 769,493). A control moment gyro (“CMG”) is a torque amplification device that uses controlled precession of stored angular momentum to produce large control torques in accordance with known laws of physics, commonly referred to as gyro dynamics. It is this torque that is used to damp roll in boat CMG installations. Ferry, Applied Gyrodynamics, Wiley (1933). The configuration and dynamics are as follows:
The angular momentum is stored in a spinning flywheel that is mounted in a one-degree-of-freedom gimbal, i.e., the spin axis of the flywheel is permitted to rotate about a gimbal axis, which is perpendicular to the spin axis and to the longitudinal axis of the boat. Usually, the spin axis of the flywheel is vertical, and the gimbal axis is athwartship, but those orientations can be reversed, so that the spin axis is athwartship, and the gimbal axis is vertical. When a boat employing a CMG rolls, conservation of the angular momentum of the flywheel causes the flywheel to rotate (or “process”) about the gimbal axis. If the precession rate is controlled, a useful gyroscopic torque is imposed about the roll (longitudinal) axis of the boat, with the net effect that rolling motion is damped. Because the torque applied to the roll axis is many times the precessional torque, it can be sufficient to damp the roll motion. The damping effect is directly proportional to (a) the rate of rotation of the flywheel, (b) the mass of the flywheel, (c) the square of the radius of gyration of the flywheel and (d) the rate at which the gyro is precessed. There are, however, limits to the amount of damping that a CMG can provide. The precession torque applied about the gimbal axis produces a reactive torque about the roll (longitudinal) axis when the spin axis of the flywheel is vertical, but as precession angle grows, and the spin axis rotates closer to horizontal, the reactive torque also produces a yawing torque, and at a full 90 degrees of precession (when the spin axis is horizontal) the reactive torque is entirely about the yaw axis.
Although the idea of using CMGs to damp roll motion of boats is almost one hundred years old, there has been very little actual use of CMGs for this application. The principal use of CMGs in modem times has been in spacecraft positioning. A few ships were outfitted with CMGs in the early twentieth century (with perhaps the last major installation being of a Sperry CMG on the Italian cruise ship Conto di Savoia in 1932), but since then fin stabilizers have replaced CMGs. More recently, Mitsubishi produced a CMG for use on small boats. In the Mitsubishi product, a passive, rotary fluidic dashpot is employed to resist precession, and air resistance is relied on for limiting flywheel rpm. U.S. Pat. No. 5,628,267 was granted to Mitsubishi for this concept of relying on air resistance to limit flywheel rpm. The patent also discloses active braking of precession although this was originally disclosed in U.S. Pat. No. 1,150,311 granted to Elmer Sperry in 1915 and to others. Because of its large size and weight for the small boats for which it is intended, the Mitsubishi product has not sold well.
Why were CMGs, which enjoyed some early success on ships, supplanted by fin stabilizers? The most probable reason is that CMGs are rate devices. They can resist roll oscillation, but they cannot resist a continuing roll angle, e.g., a sustained heel caused by a turn, a large quartering wave, or a high beam wind—all common occurrences on ships. Fin stabilizers, on the other hand, can remain deflected as long as necessary to counter a continuing heeling moment. The fact that fin stabilizers are ineffective at low (or no) speed is not usually a problem for ships because when they are in a seaway large enough to affect them, they are normally at cruise speed. Thus while CMGs were effective on ships, they appear to have been surpassed by a competing technology with broader capabilities.